Laser frequency-mixing technologies such as laser frequency doubling, sum frequency, and difference frequency are important means to implement many new wavelength lasers. By using the frequency doubling or sum frequency, a mid-infrared laser can be converted into a near-infrared laser, a near-infrared laser can be converted into visible light, or visible light can be converted into an ultraviolet laser; by using the laser difference frequency, a near-infrared laser can be into a mid-infrared laser, so that a coverage range of a laser spectral line is extended. The visible light and ultraviolet laser are widely applied to fields such as laser processing, laser medical treatment, laser display, and laser engraving. The mid-infrared laser is widely applied to fields such as atmospheric sounding, laser medical treatment, and infrared countermeasures. Therefore, the laser frequency-mixing technology plays an important role in generation of lasers of all bands.
A current mainstream laser frequency-mixing technology is implemented by using crystal frequency mixing in which nonlinear frequency doubling, sum frequency, or difference frequency is used to perform space coupling on a diode-pumped solid state laser, resulting in a complex structure of the whole system, and a large quantity of space collimation coupling commissioning needs to be performed, and mechanical stability is poor. Consequently, to ensure stable operation of the system, irregular commissioning and maintenance are required. To solve heat problems and improve stability of the solid-state laser pumping-based frequency doubling laser, sum frequency laser or difference frequency laser, laser frequency-mixing has already been performed by using a fiber laser as fundamental frequency light, which can obtain a frequency-mixing laser output characterized by high conversion efficiency, high beam quality, and simple structure. Nevertheless, the system instability problem caused by the space lens coupling between a fiber laser and a frequency doubling crystal, a sum frequency crystal or a difference frequency crystal still hinders further applications of the fiber laser frequency-mixing technology, making it difficult to obtain an all-fiber frequency-mixing laser similar to the fiber laser.